The field of the invention is open loop stepper motor control.
Stepper motors use a plurality of set positions, called steps, to provide controlled movement. Variation of current and its polarity moves a rotor of the motor through the steps, or to a fixed position at a particular step. Since the steps provide definite positioning under normal operating conditions, there is no need for feedback to verify a movement ordered by a stepper motor controller. This type of open loop operation makes stepper motors a popular choice for numerous devices. The stepper motors are an important part of many image scanner products, for example, such as multi-function peripherals (MFP) and sheet fed facsimile machines.
Feedback is often used, however, to monitor the power applied to a stepper motor. Accuracy of stepper motor positioning, and the timing with which positions are reached during motor operation both have an important affect on the operation of many such devices. Load inertia needs to be overcome, and it is often important that the rotor be accelerated and decelerated at an accurately controlled rate.
Performance issues are intertwined with cost issues in image scanning and similar devices. Size of stepper motors used in devices affects the cost of such devices. Smaller motors are more cost effective, but must be accurately controlled. This has led to careful designs of stepper motors and their controllers in image scanning devices.
Designs seek to control the rate of acceleration and deceleration to decrease the maximum torque required from the stepper motor. Lower torque motors are generally smaller and less costly. However, the smaller motor operates closer to the minimum torque required by the device and is therefore more sensitive to changes in the power supply voltage. The torque output of the motor decreases with power supply voltage
Designs must allow for variation in the power supply. A typical solution is a feedback through a sense resistor that provides information about the power applied to the stepper motor. The feedback can be used to account for power supply variations. However, the feedback arrangements are more complicated than open loop control. The open loop control must account for power variation without direct feedback from a motor power circuit, typically an H-bridge circuit. Generally, this has been addressed through motor performance specifications.
In designing the open loop control of a motor used in an image scanner, for example, steps are typically taken to ensure that the minimum required torque output for the stepper motor is met at low voltage power supply conditions. The minimum allowable torque output of the motor is measured at low voltage to determine any compensation necessary in motor control to ensure that the minimum torque is met. Voltage variations on the high end do not cause the same torque concerns, since the torque at high voltage will exceed the amount needed for device operation. Variation of power supply voltage on the high end raises other concerns, however. Under a high voltage condition, the stepper motor can exceed its thermal limits and be damaged.
Stepper motor control frequently relies upon pulse width modulation (PWM) switching to control motor voltage and current to obtain higher efficiency than linear control. PWM refers to variable on/off times (or width) of the voltage pulses applied to the transistors regulating voltage delivered to the motor. Variance in driving current delivered from transistors via PWM or another technique is the primary factor affecting the accuracy and timing of stepper motor positioning. Failure of a stepper motor can occur when driving current enters thermal limits of the motor.
A typical image scanning device using a stepper motor includes an image processing function, a print control function, and a motor control function. The separate functions are most often implemented by separate application specific integrated controllers (ASICs). The motor controller function often uses PWM. A PWM motor controller in a typical image scanning device can be programmable. The image processing function will typically include multiplexing and analog to digital conversion capability. The print control function typically includes a microprocessor.
There is a need for an improved open loop stepper motor control usable in such an image scanning device or similar device.
In a device such as an image scanning device that includes a microprocessor and a programmable motor controller, the invention compensates for a change in power supply voltage such that the motor exceeds the minimum torque required at low voltage without exceeding the maximum temperature at high voltage. An available controller input is used to sense a voltage provided by the power supply. The sensed value is used to calculate a scale factor dependent sensitivity of current in the stepper motor to the voltage provided by the power supply. Driving parameters in the programmable motor controller are then adjusted to drive the stepper motor as a function of the scale factor.
In a preferred embodiment, a scaled down version of the power supply voltage is sensed. A scale factor that depends upon the sensitivity of the motor current to the motor supply voltage is calculated by the microprocessor. Driving parameters in the controller are then adjusted to account for the scale factor. In a preferred embodiment, the driving parameters are PWM parameters, and the microprocessor multiplies the measured power supply voltage to obtain a scale factor that can then be multiplied by the nominal PWM values to obtain the new PWM values. Preferably, scale factors are different for cases of gain and attenuation.